Question

[SOLVED – See answer in bottom]

I’m trying to draw a cube with perspective, using OpenGL ES 2.0 on iOS (iPhone), but it’s appearing as a rectangular shape.

From what I’ve gathered searching the web it seems to be related to the viewport / projection matrix, but I can’t seem to put the finger on the actual cause.

If I set the viewport to a square measure (width == height) it draws perfectly well (a cube), but if I set it correctly (width = screen_width, height = screen_height) then the cube is drawn as a rectangular shape.

Should setting the Projection matrix accordingly with the Viewport make the cube stay a cube?!

My code (please let me know if more info is needed):

Render method:

// viewportSize is SCREEN_WIDTH and SCREEN_HEIGHT
// viewportLowerLeft is 0.0 and 0.0
ivec2 size = this->viewportSize;
ivec2 lowerLeft = this->viewportLowerLeft;
glViewport(lowerLeft.x, lowerLeft.y, size.x, size.y); // if I put size.x, size.x it draws well


mat4 projectionMatrix = mat4::FOVFrustum(45.0, 0.1, 100.0, size.x / size.y);

glUniformMatrix4fv(uniforms.Projection, 1, 0, projectionMatrix.Pointer());

Matrix operations:

static Matrix4<T> Frustum(T left, T right, T bottom, T top, T near, T far)
{
    T a = 2 * near / (right - left);
    T b = 2 * near / (top - bottom);
    T c = (right + left) / (right - left);
    T d = (top + bottom) / (top - bottom);
    T e = - (far + near) / (far - near);
    T f = -2 * far * near / (far - near);
    Matrix4 m;
    m.x.x = a; m.x.y = 0; m.x.z = 0; m.x.w = 0;
    m.y.x = 0; m.y.y = b; m.y.z = 0; m.y.w = 0;
    m.z.x = c; m.z.y = d; m.z.z = e; m.z.w = -1;
    m.w.x = 0; m.w.y = 0; m.w.z = f; m.w.w = 1;
    return m;
}
static Matrix4<T> FOVFrustum(T fieldOfView, T near, T far, T aspectRatio)
{
    T size = near * tanf(DEGREES_TO_RADIANS(fieldOfView) / 2.0);
    return Frustum(-size, size, -size / aspectRatio, size / aspectRatio, near, far);
}

Answer 1

Ok I found out the problem, size.x and size.y are int, so the division returns an int.

1.0 * size.x / size.y

Solves the problem.
facepalm